In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including data and signal cables for use in telephone, computer, control, alarm and related systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor. Also, the space under a raised floor in a computer room is considered a plenum if it is connected to a duct or to a plenum.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
A non-plenum rated cable sheath system which encloses a core of insulated copper conductors and which comprises only a conventional plastic jacket may not exhibit acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it ruptures either by the expanding insulation char, or by the pressure of gases generated from the insulation exposed to elevated temperature, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and generating smoke and possibly toxic and corrosive gases.
As a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. The initial cost of metal conduits for communications cables in plenums is relatively expensive. Also, conduit is relatively inflexible and difficult to maneuver in plenums. Further, care must be taken during installation to guard against possible electrical shock which may be caused by the conduit engaging any exposed electrical service wires or equipment. However, the NEC permits certain exceptions to this requirement provided that such cables are tested and approved by an independent testing agent such as the Underwriters Laboratories (UL) as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production of cable are measured using UL 910, Standard Test Method for Fire and Smoke characteristics of Electrical and Optical-Fiber Cables Used in Air-Handling Spaces. See S. Kaufman "The 1987 National Electric Code Requirements for Cable" which appeared in the 1986 International Wire and Cable Symposium Proceedings beginning at page 545. The UL 910 test is conducted in apparatus which is known as the Steiner Tunnel.
The prior art has addressed the problem of cable jackets that contribute to flame spread and smoke evolution also through the use of fluoropolymers. These, together with layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core. Commercially available fluorine-containing polymer materials have been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. However, fluoropolymer materials are somewhat difficult to process. Also, some of the fluorine-containing materials have a relatively high dielectric constant which makes them unattractive for communications media.
The problem of acceptable plenum cable design is complicated somewhat by a trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fiber be protected from transmission degradation, but also it has properties which differ significantly from those of copper conductors and hence requires special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent with a relatively low radius of curvature. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of coupling between the jacket and the core. Coupling may result because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
The use of fluoropolymers for optical fiber plenum cable jackets requires special consideration of material properties such as crystallinity, and coupling between the jacket and an optical fiber core which can have detrimental effects on the optical fibers. If the jacket is coupled to the optical fiber core, the shrinkage of fluoropolymer plastic material, which is semi-crystalline, following extrusion puts the optical fiber in compression and results in microbending losses in the fiber. Further, its thermal expansion coefficients relative to glass are large, thereby compromising the stability of optical performance over varying thermal operation conditions.
Further, a fluoropolymer is a halogenated material. Although there exist cables which include halogen materials and which have passed the UL 910 test requirements, there has been a desire to overcome some problems which still exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials exhibit undesired levels of corrosion. If a fluoropolymer is used, hydrogen fluoride forms under the influence of heat, causing corrosion. For a PVC, hydrogen chloride is formed.
In a more recently developed plenum cable, each transmission medium of a core of the cable is enclosed with a non-halogenated plastic material selected from the group consisting of a polyetherimide, a silicone-polyimide copolymer or blends of these two materials. A jacket encloses the core and is made of a non-halogenated plastic material which includes a silicone-polyimide copolymer constituent. The jacket may comprise as much as 100% by weight of the silicone-polyimide copolymer constituent.
The just-described cable is acceptable for a plenum cable having a relatively low number of transmission media. However, there is a need to provide a plenum cable which includes a relatively high number of transmission media such as, for example, at least twenty-five metallic conductor pairs.
The sought-after high number transmission media cable not only exhibits suitably low flame spread and low smoke producing characteristics provided by currently used cables which include halogenated materials but also one which meets a broad range of desired properties such as acceptable levels of corrosivity and toxicity. Such a cable does not appear to be available in the prior art. What is further sought is a cable which is characterized as having relatively low corrosive properties, and acceptable toxic properties, as well as low levels of smoke generation and one which may include a relatively high number of transmission media.